The 1st International Symposium of Inhibitors of Hematopoiesis held last year at the Pasteur Institute in Paris was the first organized international meeting devoted entirely to the systematic review of the state of research on the inhibitors of normal and leukemic hematopoiesis. Aside from a fruitful exchange of new information and generation of considerable enthusiasm amongst the participants for the further pursuit of such studies, the meeting resulted also in the publication of an informative monograph, a tentative classification of hte known inhibitors and the establishment of an International Registry for Inhibitors. The European organizers of that symposium have agreed that the 2nd International Symposium on Inhibitors be held in Providence, USA, in August 1990. A distinguished Advisory Board has been assembled and its members will give assistance in 1) the evaluation of the scientific merit of submitted abstracts, 2) delineating the most promising areas the symposium should focus on and 3) advise on the expenditure of NIH funds to cover travel expenses of such scientists as to ascertain the quality and the balance of a multidisciplinary scientific program. Our aim is to invite and/or attract a group of no more than 200-210 established scientists (and/or promising young scientists) with deep and personal interest in this area. Similarly to the 1st Symposium, the 1990 Conference will exclusively and comprehensively review the most interesting new developments in the hematopoietic inhibitors field. Tentative conference themes for each day would be: a) Cytotoxic Polypeptides, b) Soluble Inhibitory Molecules, c) Cell Surface Associated Inhibitory Molecules, and d) Retroviruses and Bone Marrow Suppression. The conference will include: 1) brief oral presentations, 2) poster sessions with discussion of the most important ones, 3) a new format of workshops (PAW), guaranteeing the participation in brainstorming sessions of each and all interested scientists, 4) the 3rd International Workshop on Classification of Inhibitors and 5) a concluding plenary session where the highlights of the presentations nad the most worthwhile conclusions of the workshops (PAWs) will be summarized and the most promising areas for future research identified.